C# 單例模式

標籤：

    一、多線程不安全方式實現

 public sealed class SingleInstance    {        private static SingleInstance instance;        private SingleInstance() { }        public static SingleInstance Instance        {            get            {                if (null == instance)                {                    instance = new SingleInstance();                }                return instance;            }        }    }

　　sealed表示SingleInstance不能被繼承。其實建構函式私人化已經達到了這個效果，私人的建構函式不能被繼承。為了可讀性，可以加個sealed。

不安全的單例指的是在多線程環境下可能有多個線程同時進入if語句，建立了多次單例對象。

   二、安全的單例模式

  public sealed class SingleInstance    {        private static volatile SingleInstance instance;        private static readonly object obj = new object();        private SingleInstance() { }        public static SingleInstance Instance        {            get            {                if (null == instance)                {                    lock (obj)                    {                        if (null == instance)                        {                            instance = new SingleInstance();                        }                    }                }                return instance;            }        }    }

　加鎖保護，在多線程下可以確保執行個體值被建立一次。缺點是每次擷取單例，都要進行判斷，涉及到的鎖和解鎖比較耗資源。

三、唯讀屬性式

 public sealed class SingleInstance    {        private static readonly SingleInstance instance = new SingleInstance();        private SingleInstance() { }        public static SingleInstance Instance        {            get            {                return instance;            }        }    }

　　 藉助readonly屬性，instance只被初始化一次，同樣達到了單例的效果。在Main函數執行第一句話之前，instance其實已經被賦值了，並不是預期的 只有到訪問Instance變數時才建立對象。

如下代碼：

 class Program    {        static void Main(string[] args)        {            Console.WriteLine("Begin");            var temp = SingleInstance.instance; ;        }    }    public sealed class SingleInstance    {        public static readonly SingleInstance instance = new SingleInstance();        private SingleInstance()        {            Console.WriteLine("初始化初始化！");        }        public static SingleInstance Instance        {            get { return instance; }        }    }

　　輸出：

在執行第一句代碼之前，執行個體已經被初始化。

解決方案是在SingleInstance中加上靜態建構函式。

 public sealed class SingleInstance    {        public static readonly SingleInstance instance = new SingleInstance();        static SingleInstance() { }        private SingleInstance()        {            Console.WriteLine("初始化初始化！");        }        public static SingleInstance Instance        {            get { return instance; }        }    }

　　在運行輸出：

   

 

四、使用Lazy

 public sealed class SingleInstance    {        private static readonly Lazy<SingleInstance> instance = new Lazy<SingleInstance>(() => new SingleInstance());        private SingleInstance(){}        public static SingleInstance Instance        {            get            {                return instance.Value;            }        }    }

　　Lazy預設是安全執行緒的。MSDN描述如下：

 Will the lazily initialized object be accessed from more than one thread? If so, the Lazy<T> object might create it on any thread. You can use one of the simple constructors whose default behavior is to create a thread-safe Lazy<T> object, so that only one instance of the lazily instantiated object is created no matter how many threads try to access it. To create a Lazy<T> object that is not thread safe, you must use a constructor that enables you to specify no thread safety.

   

五、泛型單例

    class Program    {        static void Main(string[] args)        {            Console.WriteLine("Begin");            mySingle.Instance.age = 500;            Console.WriteLine(mySingle.Instance.age);        }    }    public abstract class SingleInstance<T>    {        private static readonly Lazy<T> _instance = new Lazy<T>(() =>            {                var ctors = typeof(T).GetConstructors(BindingFlags.Instance| BindingFlags.NonPublic| BindingFlags.Public);                if (ctors.Count() != 1)                    throw new InvalidOperationException(String.Format("Type {0} must have exactly one constructor.", typeof(T)));                var ctor = ctors.SingleOrDefault(c => c.GetParameters().Count() == 0 && c.IsPrivate);                if (ctor == null)                    throw new InvalidOperationException(String.Format("The constructor for {0} must be private and take no parameters.", typeof(T)));                return (T)ctor.Invoke(null);            });        public static T Instance        {          get{ return _instance.Value;}        }    }    public class mySingle : SingleInstance<mySingle>    {        private mySingle() { }        public int age;    }

　　

C# 單例模式